1. Field of the Invention
The present invention relates to a lock-up control system for a lock-up type automatic transmission for an automotive vehicle.
2. Description of the Prior Art
Most automatic transmissions include a torque converter in a power transmission system thereof so as to increase the torque fed thereto from an engine. The common torque converters are of the type wherein a pump impeller driven by an engine rotates an operating oil within the torque converter so as to cause the rotation of a turbine runner under the reaction of a stator thereby to increase the torque (torque-converter mode). Accordingly, during the operation of the torque converter, a slip between the pump impeller and turbine runner is unavoidable. Thus, while the automatic transmission including such a torque converter in its power transmission system has an advantage in that it is easy to operate, but it also has a drawback in that the power transmission effeciency is poor and thus the fuel economy is accordingly poor. To alleviate this drawback, there has been proposed a torque converter with a direct clutch (a lock-up torque converter) wherein during relatively high vehicle speed operation, when the torque variation of an engine does not create a problem, the turbine runner is directly connected to the pump impeller (lock-up mode) thereby to eliminate slip therebetween. Some vehicles use a lock-up type automatic transmission including a torque converter of this kind.
In the conventional lock-up type automatic transmissions whenever a vehicle speed exceeds a reference vehicle speed (a lock-up vehicle speed) during operation of the vehicle in a predetermined gear ratio, a direct clutch of a torque converter is actuated to provide a lock-up state, and it is the conventional practice to set the reference vehicle speed at a fixed constant value for the gear ratio, above which a fixed constant value of torque variation of the engine does not create a problem. However, the reference vehicle speed above which the torque vibration is not regarded as a problem differs depending on a running load, such as an inclining surface of a road. The reference vehicle speed should increase with an increase in the running load. In practice, the lock-up vehicle reference speed is set at a low value on the assumption that the vehicle operates on a flat horizontal road surface. Therefore the lock-up state for a given gear ratio is effected at a relatively low vehicle speed, even with an increasing running load, thus failing to make full use of torque increase function of the torque converter. The result is that it is impossible to provide adequate acceleration to meet the requirement of a torque shortage or an unsmooth rotation of the engine results to cause vibration of the vehicle.